[alloc + set] Introducing BStackChunk API with sorting and iterator

[williamwutq]

Description: Adding BStackChunk<'a> with its iterator BStackChunkIter<'a> — fixed-stride view over BStackSlice. It holds the same semantic position as BStackSlice<'a> and offers additional atomic search/sort/select APIs (binary_search_by/binary_search_by_key/sort_by/sort_by_key/select_nth_by/select_nth_by_key).

New Feature: Yes
Important Feature: Yes
Feature Flags: alloc (+ set + atomic)
Breaking change: No
New Types: BStackChunk and BStackChunkIter
Rust Only: Yes
Fuzz: Not needed

[Copilot]

Pull request overview

Adds a new fixed-stride “record view” abstraction (BStackChunk<'a>) over BStackSlice, plus a lazy iterator (BStackChunkIter<'a>), enabling chunk-oriented iteration and APIs like binary search, stable sorting, and selection on whole records.

Changes:

• Introduces BStackChunk/BStackChunkIter with chunks/rchunks constructors (and owned-slice mirrors), iteration, and view utilities (get, as_slice, into_slice, with_stride).
• Adds chunk-level algorithms: binary_search_by(_key) (alloc-only) and sort_by(_key) / select_nth_by(_key) (gated behind set + atomic).
• Expands tests and documentation (README/CHANGELOG), and updates the roadmap (PLANNED.md) to reflect the implemented strategy and future work.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/alloc/chunk.rs	New BStackChunk / BStackChunkIter implementation, chunking constructors on BStackSlice, and chunk algorithms.
src/alloc/mod.rs	Wires in the new chunk module and re-exports the new types.
src/lib.rs	Re-exports BStackChunk and BStackChunkIter from the crate’s public API.
src/test.rs	Adds unit tests covering construction, remainder behavior, iteration, sorting, searching, selection, and trait semantics.
README.md	Documents the new types, trait semantics, and the chunked-view API surface.
CHANGELOG.md	Adds release notes describing the new BStackChunk API and behavior/traits.
PLANNED.md	Updates the design notes to focus on future external-merge-sort and out-of-core selection strategies for chunks.

Suppressed comments (1)

src/alloc/chunk.rs:567

• ExactSizeIterator requires that len() be exact, but for BStackChunkIter the number of remaining chunks is derived from a u64 length and can exceed what fits in usize on 32-bit platforms. With the current implementation this can silently truncate via size_hint and violate the ExactSizeIterator contract. Consider only implementing ExactSizeIterator where it is always representable (e.g. gate it on 64-bit pointer width) or remove the impl.

impl<'a> ExactSizeIterator for BStackChunkIter<'a> {}

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